1. Field of the Invention
The present invention relates to an apparatus for purifying an exhaust gas of an engine, particularly, to the apparatus including a catalyst of a NOx absorbing-and-reducing type for purifying NOx-including exhaust gas that is emitted during a lean operation.
2. Description of the Prior Art
A three-way catalyst included in an engine for a vehicle or the like efficiently purifies harmful components, such as CO (carbon monoxide), HC (hydrocarbon), and NOx (nitrogen oxides). However, since a window is limited to a narrow range in the vicinity of a theoretical air fuel ratio (λ=1), a NOx purification ratio is lowered in a recently developed engine, such as a so-called lean burn engine, which employs a direct-injected stratified combustion method for the purpose of improving fuel consumption performance. To overcome this problem, a catalyst of a NOx absorbing-and-reducing type (which hereinbelow may simply be referred to as a “NOx catalyst”) is included in an exhaust passage. At an air fuel ratio representing an overoxygen atmosphere, the NOx catalyst absorbs the NOx component included in exhaust gases, reduces the NOx component absorbed according to a reduced oxygen concentration, and emits it.
When a lean operation continues for a long time, the absorbed NOx catalyst is saturated. Therefore, when the amount of the absorbed NOx is equal to or greater than a predetermined absorption amount, the air fuel ratio of the exhaust gas is adjusted to be rich to emit NOx from the catalyst, thereby allowing the NOx catalyst to regain the absorbing capability.
However, the NOx absorption amount, which is employed as a determination criterion for starting the aforementioned NOx emission processing, cannot be directly measured. Generally, the amount is obtained through estimation. In a typical example of the estimation, a momentary NOx absorption amount is estimated according to the operation condition in units of a predetermined cycle (for example, a control cycle or a sampling cycle), and an integrated NOx absorption amount obtained through integration of the momentary amounts is employed as a NOx amount. In this case, estimation accuracy lower and an error in the estimated amount causes various defects.
For example, when an estimated NOx absorption amount is greater than an actual amount, a case can occur in which NOx emission processing is early started while NOx is not absorbed so much as to be emitted. Also, a case can occur in which a NOx emission processing needlessly continues while NOx is completely emitted. Neither of the cases is preferable. On the other hand, when an estimated NOx absorption amount is less than an actual amount, a case can occur in which NOx emission processing does not start while NOx is absorbed sufficient to be emitted. Also, a case can occur in which NOx emission processing early stops while NOx is not yet completely emitted. Neither of the cases is preferable in terms of catalyst performance.
Under the present circumstances, various proposals have been made for techniques that allow an accurate estimate of the NOx absorption amount to be obtained. For example, as is disclosed in the Japanese Laid-open Patent Publication No. 7-139340, the NOx absorption amount is added in lean operation, and the NOx absorption amount is subtracted in either rich operation or theoretical air fuel ratio operation. In addition, the addition amount (momentary NOx absorption amount) is estimated according to the engine speed, an intake air pressure, and the like; and the subtraction amount (momentary NOx emission amount) is estimated according to, for example, an over-fuel-supply amount and a catalyst temperature.
In estimation techniques proposed to date, a significantly great deviation that cannot be ignored still occurs between an estimated amount and an actual NOx absorption amount, wherein matters still remains for improvement. As a cause for the deviation, important parameters that influence the increase/decrease in the NOx absorption amount are considered to exist in addition to parameters used to estimate a current NOx absorption amount.